1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same. The present invention can be suitably applied to an electronic camera such as a video camera or a digital still camera, a film camera, and a broadcast camera etc.
2. Description of the Related Art
In recent years, image pickup apparatuses (or cameras) such as video cameras, digital still cameras and surveillance cameras using a solid state image pickup element are required to have many functions and to be small in their entire size.
In accordance with such requirements, optical systems (image pickup optical systems) used in such cameras are required to be constructed as zoom lenses that are small in size, have a wide angle of view while having excellent optical performance.
When a color separating optical system such as a prism or filter is used in an image pickup apparatus, the color separating optical system is provided in its image side end portion. Accordingly, zoom lenses used in such image pickup apparatuses are required to have back focus equal to the optical path length of the color separating optical system.
As a zoom lens that meets the above requirements, a four-unit zoom lens having a positive first lens unit, a negative second lens unit, a positive third lens unit and a positive fourth lens unit arranged in the mentioned order from the object side to the image side is known. Note that a lens unit or element having a positive optical power or positive refractive power is referred to as a positive lens unit or element in this specification, and a lens unit or element having a negative optical power or refractive power is referred to as a negative lens unit or element.
One known type of such a four-unit zoom lens is what is called a rear-focus four-unit zoom lens in which the second lens unit is moved to effect zooming and the fourth lens unit is moved to correct image plane variations caused during zooming and to effect focusing.
There have been disclosed various rear-focus four-unit zoom lenses that have the first lens unit including a negative front subunit and a positive rear subunit arranged in the mentioned order from the object side to the image side to achieve a wide angle of view (see Japanese Patent Application Laid-Open Nos. H11-287952 and H11-287954).
A five-unit zoom lens having a positive first lens unit, a negative second lens unit, a positive third lens unit, a positive fourth lens unit and a positive fifth lens unit arranged in the mentioned order from the object side to the image side is also known as a zoom lens that meets the above described requirements.
In a known type of such a five-unit zoom lens that has a relatively wide angle of view, the second lens unit and the fourth lens unit are moved for zooming, and the fourth lens unit is moved for focusing (see U.S. Pat. No. 5,694,252 and Japanese Patent Application Laid-Open No. 2002-365539).
When a zoom lens is constructed as a rear focus system, the size of the entire lens system can be made smaller, focusing can be made faster and short distance shooting can be made easier as compared to zoom lenses in which focusing is effected by the first lens unit.
On the other hand, however, the rear focus zoom lens suffers from larger aberration variations during focusing.
It is difficult to achieve a wide angle of view and a high zoom ratio while achieving excellent optical performance throughout the entire zoom range.
To achieve a wide angle of view and a high zoom ratio in a rear-focus zoom lens while achieving excellent optical performance throughout the entire zoom range, it is necessary to design the configuration of each lens unit, especially the configuration of the first lens unit appropriately.
In addition, to provide back focus long enough to allow to provide a color separating optical system on the image side of the zoom lens, it is necessary to design the refractive power arrangement and the lens configuration in each lens unit appropriately.
In particular, in the case of the above described types of four-unit zoom lenses and five-unit zoom lenses, it is very difficult to enlarge the angle of view and increase the zoom ratio while achieving excellent optical performance throughout the zoom range unless the lens configuration of the first lens unit is appropriately designed.